Hi Tweedy,
I'll pretty much second everything JohnK had to say. Keep in mind that neither the VP100 or the VP150 is going to sound exactly like the M60s. Just the difference in cabinet size will make a difference. The question shouldn't be "does my center channel sound just like my mains"? It should be "does my center channel do it's job? Do voices sound reasonably natural? Can I easily understand the dialog in most cases (sometimes the source is the problem, and not the speaker)?"
If you're not satisfied with the answers, in your receiver's speaker setup menu, try raising the level of your VP100 by just 3dB, and see if that helps. This is allegedly the smallest increment human hearing can detect, and I run my center 3 dB hotter than the my mains. In my room, and my setup, I feel it helps intelligibility, without spoiling pans across the front 3. YMMV.
I also agree with John when it comes to using your receiver's processing capability. There are I few instances when I prefer listening in stereo, but by and large, I like the spacial quality the processing adds. MANY feel differently, and much prefer to listen in stereo only. Give it a try and see how YOU feel.
